As the world's demand for fossil fuels increases, energy companies find themselves pursuing hydrocarbon resources in more remote areas of the world. Such pursuits often take place in harsh, offshore conditions. In recent years, drilling and production activities have been started in the Arctic regions.
Spill detection and mapping may be particularly important for Arctic spills as oil may be hidden from view under snow and ice during periods of almost total darkness. Close to 24 hours of daylight in the spring and summer months facilitates monitoring oil spilled during the break-up and open water periods, but periods of fog and a low cloud ceiling remain as serious impediments. During freeze-up and through much of the winter, long periods of darkness and multiple oil/ice scenarios add to the challenges of detection, mapping and tracking oil in ice.
One approach for detecting an oil mass trapped beneath or within a solid ice sheet or on the ice surface under snow is based on the use of a low level airborne ground penetrating radar (GPR). In a paper titled “Remote Sensing for the Oil in Ice Joint Industry Program 2007-2009” by Dickins et al., a commercially available GPR system in the 500 MHz to 1 GHz frequency range is described that may be operated both from the ice surface and at low altitude from a helicopter to detect oil layers in the 1-3 cm range trapped in relatively smooth ice.
GPR is sensitive to the presence of oil in the snow pack over a broad range of snow densities and oil types. Oil located at the base of the snow tends to reduce the impedance contrast with the underlying ice or soil substrate resulting in anomalously low amplitude radar reflections and thereby enhances the prospects for detection with GPR. Sea ice, on the other hand, has a much higher electrical conductivity that varies substantially both laterally and vertically and can exhibit a high degree of anisotropy due to preferred crystal alignment. GPR may provide reliable thickness measurements for low salinity ice, but significant signal attenuation occurs for high-salinity first-year ice. Consequently, the problem of detecting an oil mass is simpler to formulate for dry snow than it is for sea ice since snow has a relatively isotropic structure and low conductivity.
Direct spill detection from SAR satellites and airborne SLAR/SAR systems is relatively straightforward for large spills in very open drift ice. However, detection of an oil mass covered by ice is much more difficult. Moreover, during freeze-up in fall and early winter, any detection of oil among ice with SAR/SLAR airborne or satellite sensors may be complicated by the presence of grease ice. Grease ice is the earliest smooth stage of ice crystals at the water surface. The presence of grease or new ice in conjunction with an oil spill on the water will produce close to identical signatures in the radar imagery, making detection of an oil slick difficult or impossible.
Other technologies that may be used to detect Arctic oil spills or leakages include forward looking infrared (FLIR) systems, SONAR systems, and hyperspectral imaging systems. In some cases, trained dogs may be used to reliably detect oil near the surface of the ice. A thickness of the ice in Arctic regions, for example, may vary from a few centimeters to 5 meters. While these other technologies may work when the oil is on or near the surface of the ice, they may not be very effective in detecting an oil mass covered by thick ice.
Yet another approach for detecting an oil mass under ice is based on nuclear magnetic resonance (NMR), as disclosed in U.S. Published Patent Application No. 2011/0181279. In this approach, a volume of substances is located under the surface, wherein at least a portion of the volume of substances is within a static magnetic field. At least one radio-frequency excitation pulse is transmitted from a remote location above the volume of substances to generate a nuclear magnetic resonance (NMR) signal from the volume of substances. From the remote location, the NMR signal from the volume of substances is detected. The detected NMR signal is processed to determine whether the volume of substances includes the liquid. Even in view of NMR, there is still a need to improve upon the detection of an oil mass covered by ice.